1. Field of the Invention
The present invention relates to a semiconductor device having a circuit composed of a thin film transistor (hereafter referred to as TFT), and to a method of manufacturing thereof. For example, the present invention relates to an electro-optical device, typically a liquid crystal display panel, and to electronic equipment loaded with that kind of electro-optical device as a part.
Note that, throughout this specification, the term semiconductor device specifies devices in general which function by utilizing semiconductor characteristics, and that electro-optical devices, semiconductor circuits, and electronic equipment are all semiconductor devices.
2. Description of the Related Art
Techniques of forming thin film transistors (TFTs) using a semiconductor thin film (with a thickness on the order of several tens to several hundreds of nanometers) formed on a substrate having an insulating surface have been in the spotlight in recent years. Thin film transistors are widely applied in electronic devices such as an IC or an electro-optical device, and in particular, their development as a switching element of a pixel display device has been accelerating.
For example, a liquid crystal display device is a device in which a TFT is placed in each of several million pixels laid out in a matrix shape, and by controlling the electric charge applied to each pixel electrode by the switching function of the TFTs, the electro-optical characteristics of a liquid crystal is changed, and image display is performed by controlling the light transmitted through a liquid crystal panel.
An IPS method (recorded in Japanese Patent Application Laid-open No. Hei 6-160878) of controlling an electric field in the horizontal direction, with respect to a substrate, in a parallel electrode structure, is known as a method of driving this type of liquid crystal display device.
A liquid crystal display device driven by this IPS method can be driven at a low voltage, and compared to other driving methods (such as a TN method or a STN method), the IPS method has a characteristic of a larger viewing angle.
Further, an IPS liquid crystal display device has a TFT, a gate line, a source line, a pixel electrode, a common line, and a common electrode, extending from the common line in a pixel region, on the same substrate. In order that the electric field applied to a pixel electrode does not impart any influence on another pixel, each pixel electrode is structured as sandwiched by the common electrode arranged in parallel to the pixel electrode. The electrode surface area necessary for these electrodes therefore reduces the aperture ratio of the IPS liquid crystal display device.
Furthermore, in order to ensure the electric charge retention time in a general liquid crystal display device, it is necessary to form a storage capacitor. A sufficient electrode surface area is necessary in order to form a storage capacitor also in the IPS liquid crystal display device, and therefore the aperture ratio is reduced.
In addition, if the wirings and the electrodes are made minuter in order to increase the aperture ratio, then it becomes difficult to ensure a sufficient storage capacitance.